An objective lens which is one of optical members for use in an optical head is disposed such that by controlling the position thereof along an optical axis and in a perpendicular direction to the optical axis, a laser beam can be adjusted to focus on an accurate spot on a track of a recording medium. In the arrangement, the objective lens is secured onto an objective lens support member with adhesive, which is movably supported to a base plate. Generally, the objective lens support member is adjusted in its weight balance by a balance weight provided therein so as to maintain its stable neutral position and to allow its stable shifting without tilting the optical axis. Further, depending on the movable supporting structure of the support member, the balance weight is sometimes designed to be ring-shaped and disposed in such a position as to be coaxial with the objective lens in order to bring its center of gravity to the optical axis of the objective lens. Furthermore, in some of those systems, in order to avoid unnecessary contact of the surface of the objective lens with other members and the like, a ring-shaped lens guard member is attached thereto. The ring-shaped balance weight and lens guard member, or the member having both of the functions are secured onto the support member with adhesive as well as the objective lens.
Referring to FIG. 6 and FIG. 7, the following description will discuss a concrete example of the prior art with respect to the above structure.
As shown in the drawings, an objective lens 21 is secured inside a ring member 22 serving as a balance weight and a lens guard member by the use of adhesive, and the ring member 22 is further secured on a support member 23 by the use of adhesive. As shown in FIG. 6, the support member 23 is fixed to a base plate (not shown) through a pair of focusing direction use plate springs 24a, 24b, a junction member 25, a pair of tracking direction use plate springs 26a, 26b and a pair of supporting blocks 27a, 27b, and is allowed to move in directions indicated by F (for focusing) and T (for tracking) in FIG. 6 by the function of the plate springs 24a, 24b, 26a, 26b. A focusing coil 28 and a pair of tracking coils 29a, 29b are also fixed to the support member 23, and those coils permit it to be shifted in the direction F or T by their interaction with a magnetic circuit fixed to the base plate (not shown).
Meanwhile, in the above holding structure with respect to objective lens, both the objective lens 21 and the ring member 22 are secured to the support member 23 so as to be coaxial, and in that case two processes of adhesive bonding are necessary between the objective lens 21 and the ring member 22 as well as between the ring member 22 and the support member 23, thereby requiring time-consuming work in those processes. Moreover, especially in the adhesion of the objective lens 21 and the ring member 22, since a surface-to-surface adhesion is performed between comparatively large areas, usable adhesives are limited and skilled workers are required in the process.